The increased emphasis on improving power density and first cost of internal combustion engines has led to increased running speed, increased piston stroke and increased peak firing pressures of combustion with in the cylinders. These improvements tend to place the crankshaft under increasingly higher stress and vibratory torque.
Increased running speed places the internal combustion engine crankshaft under higher stress due to higher inertial loading as well as bringing previously excluded natural modes of vibration within the new higher operating speed range of the engine.
Increasing the piston stroke of the engine enlarges the engine's displacement and its power output. However the crankshafts pins or throws must be set further from the central supporting main journals to achieve the increased stroke. The lengthening of the crankshaft support geometry to carry pin loads makes the crankshaft more flexible and weaker.
In the past only very large internal combustion engines with piston bore diameters above 300 millimeters were under much risk from crankshaft failure due to axial vibration. Crankshaft axial vibration is a mode of vibration resulting from the crankshaft expanding and compressing along its axis of rotation. This mode of vibration affects the crankshaft cheeks or pin throws. The cheek is that part of the crankshaft connecting the main journals with the eccentrically mounted pins. The U shape of the cheeks and pins extending from the main journals make the cheeks susceptible to vibration toward and away from the middle of the "U's", much like a "U" shaped tuning fork. Typically the comparatively short, stiff and low weight crankshafts of engines with less than 300 millimeter piston bores lacked problems with axial modes of vibration. The stiffness and light weight of heretofore made small crankshafts predetermined them to have comparatively high natural frequencies, which were well above frequencies excited at the engines highest operating speed.
Internal combustion engine crankshafts must also endure high torsional vibrations. Since internal combustion engines do not produce smooth power and experiences positive and negative speed fluctuations within each revolution of the crankshaft. Normally the engine is equipped with a flywheel to smooth the torque and speed fluctuations, which would otherwise be more significantly present in the engine drive line. When the engine revolves subcomponents and segments about the crankshaft centerline as well as subcomponents and segments of any driven piece of equipment, there is always certain rotational flexibility between successive rotating masses. The flexibility existing between the rotating masses of the engine drive line allow for slight angular deflections to propagate among, between and through the engine drive line. This angular vibration makes the internal combustion engine experience torsional vibrations between the drive line masses.
Torsional vibrations and/or axial vibrations present within an internal combustion engine may not be acceptable. As these vibrations may lead to infinitely high stress resonance conditions, which cause parts to catastrophically fail or may be unacceptable for customer stipulated reasons or perceptions.
British Patent 1,016,914 describes a cylinder connected to the engine block and having pressurized lubrication oil supplied to both sides of a piston disposed in the cylinder and connected to the crankshaft to dampen axial vibrations.